The present invention relates to a calendering arrangement for a paper machine for producing higher-gloss paper grades, in particular paper grades with a Hunter gloss % above 40.
Calendering is a method by means of which the properties, in particular the thickness profile, smoothness, gloss and surface porosity of a web-like material, such as a paper web, are sought to be generally improved. In calendering the paper web is passed into a nip which is formed between rolls pressed against each other and in which the paper web is deformed by the action of temperature, moisture and nip load, in which connection the physical properties of the paper web can be affected by controlling the above-mentioned parameters and the time of action. The good physical properties attained by calendering lead to better print quality, thereby bringing a competitive advantage to the manufacturer of paper. Paper grades that contain a small amount of coating are environmentally more friendly than abundantly coated grades. The raw material costs of paper are formed roughly such that the less chemical pulp and the more mechanical pulp and fillers there are in paper, the cheaper the paper. The combustibility of uncoated paper grades is also considerably better as compared with coated grades, and thus it is possible to use paper waste as a source of energy and thereby to avoid the recycling costs which are today still rather high.
Long-nip calendering has been found in practice to be good for producing low-gloss paper grades, i.e. grades having a Hunter gloss % below 40, by calendering. When higher-gloss is required, the nip load in long-nip calendering is not sufficient to provide gloss. With ever-increasing running speeds, calendering is becoming a bottleneck in the papermaking process, and satisfactory quality is not achieved by today's machine calendering units. Some of the drawbacks of the present papermaking process are also that    the loss of bulk increases when gloss is improved,    in order to provide sufficient quality of gloss, it is necessary to use webs with an abundance of coating and/or to use off-line calendering, in particular  multi-nip supercalendering and/or soft calendering, and    investment costs and space requirement are high.
a. Machine calendering means here and hereafter calendering in a calendering unit in which nips are formed between non-resilient smooth-surface metal rolls placed one upon the other. The length of the nip in a machine calendering unit is dependent on the diameter of the rolls and on the thickness of the paper web that is being calendered, being typically very small, wherefore the nip load in the nips is relatively high.
b. Supercalendering, which provides in off-line operation in practice the best result in terms of quality, means above and hereafter calendering in a calendering unit in which nips are formed between a smooth-surface roll, such as a metal or chilled-surface roll, and a roll covered with a resilient coating, such as a paper or polymer roll, in which connection a remarkably wide nip is formed. The resilient-surface roll conforms to the contours of the surface of paper and presses the opposite side of paper evenly against the smooth-surface metal roll. Because of the resilient-surface roll, the calendering time is also longer, wherefore in order to assure adequate capacity, the paper machine must be equipped today even with three supercalenders. In one supercalender there are today typically 10 to 12 nips. For the purpose of treating both sides, the calender comprises a so-called reversing nip in which there are two resilient-surface rolls placed against each other. The linear load in the supercalender increases from the top nip to the bottom nip because of the force of gravity, but by using relieving of the rolls, this increase in load can be compensated for.
c. Soft calendering means above and hereafter calendering in a  calendering unit in which nips are formed in a manner similar to that of a supercalender between a smooth-surface metal roll and a roll covered with a resilient coating, in which connection a nip of substantial width is formed. In a soft calender, each nip is formed between separate pairs of rolls, so that the nip load in the individual nips can be adjusted in each individual nip. The machine calenders of a paper machine can be replaced with soft calenders. In order to treat both sides of the web in the calender, the positions of the rolls in successive nips are exchanged so that both sides of the web are treated in the same way.
d. Long-nip calendering means above and hereafter calendering in a calendering unit in which a nip is formed between a roll provided with a resilient shell, the shell of said roll being made, for example, of polyurethane, and a press roll, shoe roll which is provided with inside loading shoes and which is made of metal. One long-nip concept marketed by Metso Paper, Inc. is called OptiDwell™, which includes two different long-nip calenders:                i. OptiDwell Shoe™ calender based on shoe press technology,        ii. OptiDwell Belt™ calender based on roll/belt technology.        